Many catalysts and processes are known for the preparation of olefin polymers. Ziegler-Natta (ZN) catalyst compositions and chromium oxide compounds have, for example, been found to be useful in the preparation of polyolefins.
Further the use of metallocene catalysts in olefin polymerisation has been known for many years and has been found to afford polymer properties not easily available by using ZN-catalysts. Metallocene compounds/procatalysts are conventionally activated using a cocatalyst such as an aluminoxane known from the literature to form the active metallocene catalyst species.
The first single-site catalysts to be developed were homogeneous, i.e. they were used in solution in the polymerisation reaction. Due to the many drawbacks of homogeneous solution systems, several different approaches have been used to try to overcome the problems of the solution catalyst systems. Nowadays the widely used catalyst systems comprise heterogeneous catalysts, wherein catalyst components are supported on an external carrier. Such catalyst systems are described for example by Severn et al., Chem. Rev. 2005; 105(11); 4073-4147 or in the Handbook Tailor-Made Polymers: Via Immobilization of Alpha-Olefin Polymerisation Catalysts of Severn et al.
The carriers used have a porous structure in order to facilitate catalyst impregnation of the support. Carrier materials are typically polymeric or inorganic supports, most typically silica, alumina or magnesium dichloride based materials.
However, the use of an external support involves additional costs, the quality of the support must be carefully controlled and polymers made using supported catalysts can contain carrier residues which cause some problems.
For example, in film applications this is an important feature, since for polymers made by using such supported catalysts, the carrier residues may be visible in the film.
In recent years solid metallocene catalyst systems, providing the advantages of both homogenous and heterogeneous catalysts, were developed by using an emulsion/solidification technology for their preparation without using an external carrier, as for example disclosed in WO 03/051934. Such catalyst systems are further featured by spherical particles with low porosity.
The preparation of this kind of catalyst systems by using an emulsion/solidification technology is based on a liquid/liquid emulsion system comprising at least two phases, whereby the catalyst particles are separated out of the dispersed phase of the emulsion via solidification.
As is disclosed in WO 03/051934 such a process comprises the formation of an emulsion, wherein the continuous phase, in which a solution of the catalyst components forms the dispersed phase in the form of droplets, is immiscible with said catalyst component solution and is selected from halogenated organic solvents, and subsequent solidification of said droplets, comprising the catalyst components, dispersed in a continuous phase of the formed emulsion.
According to the description of WO 03/051934 the continuous phase preferably comprises a halogenated organic solvent, particularly a fluorinated organic solvent and/or a functionalized derivative thereof, still more preferably the solvent comprises a semi-, highly- or perfluorinated hydrocarbon and/or a functionalized derivative thereof.
It is in particular preferred, that said solvent comprises, preferably consists of, a perfluorohydrocarbon or a functionalized derivative thereof, preferably C3-C30-perfluoroalkanes, -alkenes or -cycloalkanes, more preferred C4-C10-perfluoroalkanes, -alkenes or -cycloalkanes, particularly preferred perfluorohexane, perfluoroheptane, perfluorooctane or perfluoro (dimethylcyclohexane) or a mixture thereof.
According to the method described such catalyst systems are prepared by emulsifying the catalyst solution into cold fluorous hydrocarbons, like perfluorooctane (PFO) or perfluoro-1,3-dimethylcyclohexane (PFC) and then solidifying the droplets by mixing the emulsion with hot PFO or PFC. Solidified particles are then separated from PFO or PFC and dried with inert gas flow. As PFO and PFC are very easily evaporated some of the solvent might be lost in this process, and in handling point of view are demanding chemicals. Furthermore PFO and PFC are quite expensive solvents so it is not desired to lose any of it during the process.
In addition, in order to obtain such an emulsion, and especially also to preserve the droplet morphology during the solidification step, the use of a surfactant is essential.
According to the description of WO 03/051934 the surfactant is preferably based on hydrocarbons (including polymeric hydrocarbons with a molecular weight e.g. up to 10 000) optionally interrupted with (a) heteroatom(s), preferably halogenated hydrocarbons optionally having a functional group, preferably semi-, highly- or perfluorinated hydrocarbons as known in the art.
Alternatively and as shown in cited prior art documents, the surfactant is prepared in-situ by reacting a surfactant precursor with a compound of the catalyst solution. Said surfactant precursor may be a halogenated hydrocarbon with at least one functional group, e.g. a highly fluorinated C1 to C30 alcohol having at least one functional group selected from —OH, —SH, —NH2, —COOH, —COONH2, oxides of alkenes, oxo-groups and/or any reactive derivative of these groups, which reacts e.g. with a cocatalyst component, such as aluminoxane.
The use of surfactants improves essentially the preparation process and has clear benefit for the catalyst morphology. However, in some cases surfactants might be not effective enough in stabilizing the emulsion and consequently the morphology is not preserved on the desired level. In addition controlling of the processes for in-situ formation of the surfactant might be demanding due to the many factors effecting the final result.
In addition, the use of such highly reactive fluorinated compounds is considered problematic from HS&E (Health, Safety & Environment) point of view, so that the availability of these compounds in the future is not guaranteed.
Many prior art catalyst systems have the drawback that they tend to dissolve to some extent in the polymerisation medium needed in the slurry reactors and thus the morphology of the catalyst systems is reduced, which in turn cause fouling and sheeting in the reactors.
To solve the above mentioned disadvantages of solubility of the catalyst systems highly controlled catalyst pre-polymerisation as part of the catalyst preparation are suggested in the state of the art. In addition to being a sensitive step, prepolymerisation increases costs on a commercial scale.
Thus there is a strong need to develop improved methods for preparing solid metallocene catalyst systems, however without any external support, with the emulsion/solidification technology using more sustainable materials in environmental, safety and healthy point of view, which overcome the problems of the catalyst systems according to the state of the art.
It was therefore an object of the invention to provide an improved and suitable process for preparing solid metallocene catalyst systems with the emulsion/solidification technology, without the need of external support materials, which catalysts render possible to produce polymers in an efficient manner, i.e. using solid catalyst systems which are less soluble in the polymerisation media and are obtained by a preparation process in which environmentally more friendly, respectively safer materials can been used. A further object is that the obtained catalyst system enables to produce high bulk density polymers with narrow particle size distribution in an efficient manner.
The finding of the present invention is that the solid catalyst system must be produced based on emulsion/solidification technology in which a liquid clathrate constitutes the dispersed phase of the emulsion and the solvent used for the continuous phase of the emulsion is a nonreactive fluorinated synthetic oil having a viscosity at 20° C. according to ASTM D445 of at least 10 cSt. It is a particular finding of the present invention that the use of a non-reactive fluorinated synthetic oil as the continuous phase avoids the addition of a separate surfactant and enables to produce solid catalyst systems with a very narrow particle size distribution.
Thus the present invention is therefore directed to a process for the preparation of a solid olefin polymerisation catalyst system, comprising an organometallic compound of a transition metal of Group 3 to 10 of the Periodic Table (IUPAC 2007) in the form of solid particles comprising the steps of    I) generating an emulsion by dispersing a liquid clathrate in a solvent (S)            wherein        (i) the solvent (S) constitutes the continuous phase of the emulsion and comprises a nonreactive fluorinated synthetic oil having a viscosity at 20° C. according to ASTM D445 of at least 10 cSt up to 2000 cSt        (ii) the liquid clathrate constitutes in form of droplets the dispersed phase of the emulsion,            II) solidifying said dispersed phase to convert said droplets to solid particles and    III) optionally recovering said particles to obtain said catalyst system,wherein the liquid clathrate comprises            (a) a lattice being the reaction product of                    a1) a transition metal compound of formula (I)LmRnTXq  (I)                            wherein                “T” is a transition metal of anyone of the groups 3 to 10 of the periodic table (IUPAC 2007), preferably a transition metal of anyone of the groups 4 to 6 of the periodic table (IUPAC 2007), more preferably titanium (Ti), zirconium (Zr) or hafnium (Hf), i.e. zirconium (Zr) or hafnium (Hf),                each “X” is independently a monovalent σ-ligand,                each “L” is independently an organic ligand which coordinates to the transition metal (T),                “R” is a bridging group linking said organic ligands (L),                “m” is 2 or 3, preferably 2,                “n” is 0, 1 or 2, preferably 1,                “q” is 1, 2 or 3, preferably 2,                m+q is equal to the valency of the transition metal (T),                                    a2) a cocatalyst comprising aluminoxane            a3) a compound being effective to form the lattice with the transition metal compound and/or the aluminoxane and                        b) a hydrocarbon solvent (HS).        
Thus, an additional object of the invention is the use of solvent (S) as defined here below in the formation and stabilisation of the emulsion used in the catalyst system preparation in combination with liquid clathrates.
It is especially notable that the specific selected solvents (S) used in the preparation of the catalyst system according to the present invention, are e.g. in HS&E point of view, more convenient materials.
In addition it should be noted that the specific solvent (S) used according to the present invention is not volatile, which makes it much easier and safer to handle.
A further finding of the present invention is that very stable emulsions are formed without the need of adding a separate surfactant and that spherical catalyst particles showing a narrow particle size distribution can be produced.
As an additional advantage it can be mentioned that the process according to the present invention can be carried out in a simplified way, e.g. in only one vessel and with a simplified solidification step, as disclosed in more detail below.
Therefore, the present inventive preparation method represents an economically more attractive alternative for the preparation of such kind of catalyst systems as described here.